In hadron therapy, a beam of particles composed of quarks, such as protons or positive ions, e.g. carbon ions, is used to selectively irradiate tissue, for example for the treatment of cancer. Such particles may inflict damage to the tissue cells, e.g. particularly to the DNA in these cells. Because of a reduced ability to repair damaged DNA, cancerous cells are known to be particularly vulnerable to such attack. One of the advantages of particle therapy over conventional external beam radiation therapy, e.g. using high-energetic photon irradiation, is the ability to obtain a good localisation of the released energy. Although bremsstrahlung X-rays may penetrate more deeply into the tissue, the absorbed dose in the tissue exponential decays with increasing depth. For protons and heavier ions, the dose increases while the particle penetrates the tissue and loses energy continuously, such that the dose increases to a depth corresponding to the energy-specific Bragg peak near the end of the particle's range. Beyond the Bragg peak, a steep drop to zero or near zero occurs. Thus, by carefully planning the treatment, less energy may be deposited into healthy tissue surrounding the target tissue.
In a known hadron therapy system, the radiation beam may be generated by a charged particle accelerator, such as a cyclotron, synchrocyclotron or synchrotron. The energy of the particle beam, which determines the depth of penetration, e.g. the Bragg peak depth, may be adjusted to the desired range by an energy degrader and selector system. A beam guidance system may further direct the particle beam to a therapy room, in which a patient may be positioned on a therapy couch. A beam delivery system may then deliver the beam to the patient in accordance with a treatment plan. Such beam delivery system may be a fixed beam delivery system for delivering the beam to the patient from a fixed irradiation direction, or may be a rotatable beam delivery system capable of delivering the beam to the patient from a plurality of irradiation directions.
Prior to irradiation, the patient position may be accurately determined and adjusted in order to align the target tissue with the particle beam in accordance with a treatment plan. In order to deliver a spatial dose distribution in the patient which conforms well to a treatment plan defining the target distribution of dose, it is known in the art to use spot or pencil beam scanning In spot or pencil beam scanning systems, the charged particle beam is deflected in a raster scanning pattern, e.g. similar to the manner in which a television image is constructed in a cathode ray tube television. Thus, a pixelated or continuous approximation to the dose delivery plan may be painted in the target volume by modulating the beam intensity or the scanning speed as function of the scanning position. Furthermore, by varying the beam energy, a depth dimension may be added to the dose delivery by iterating the raster scanning process over a plurality of layers defined by different beam energies.
However, the electromagnets in a pencil beam scanning system may be large, heavy and costly. Furthermore, a rotatable beam delivery system may comprise a gantry for selecting a treatment angle with respect to the patient, e.g. for rotating the direction of propagation of the treatment beam around a longitudinal axis of the patient. Since at least part of the pencil beam scanning system may be implemented on such rotatable gantry, weight and size of the pencil beam scanning system components may further increase the cost and size of the overall system, e.g. the gantry.
Therefore, a need exists for reducing the size and weight of pencil beam scanning systems in charged hadron radiation therapy. Unfortunately, reducing the size and weight may also imply a smaller treatment field size, e.g. which is too small for treating targets having a diameter larger than, for example, 5 cm.